Pickering design flaw: Like engine accelerating when you step on the brakes

The separation of coolant and moderator has created a positive void coefficientof reactivity — an undesirable characteristic from a reactor safety point ofview, unique to pressure-tube reactor designs such as the CANDU and theRBMK, the Russian design used at Chernobyl.

A positive void coefficient of reactivity means that the nuclear chainreaction speeds up whenever there is a loss of coolant accident. This isundesirable because under adverse circumstances, a loss of coolant accidentmay be compounded by a loss of regulation as well (a power surge).

When the coolant is lost in a CANDU reactor, the moderator remainsintact, so the nuclear reaction increases because there is an increasedavailability of thermal neutrons when steam forms in the primary coolingcircuit.

So, under accident conditions, the heat increases in the nuclear corecreating more power, not less. Thus a positive void coefficient of reactivityis a much more dangerous design, like having a car engine accelerate at thesame time one is trying to apply the breaks.

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About the OCAA

The Ontario Clean Air Alliance (OCAA), established in 1997, successfully led the campaign to phase out the use of dirty coal power in Ontario. We are now working to move our province to a 100% renewable electricity system.